Compact optoelectronic continuous wave terahertz spectroscopy system (230–400 GHz) for paper sorting and characterization

Continuous wave (CW) THz spectroscopy, exploiting the 1.55 μο telecom wavelength and technologies [1], promises diverse beneficial applications in, e.g., medical imaging, industry, security, and non-destructive material testing. However, in order to enable these complex applications, compact and novel spectroscopy systems, based on cost-effective and non-complex techniques, have to be developed. Accordingly, the carrier frequency, the wave-shape, and the bandwidth can be adapted to the measurement's environment and material properties. By utilizing the optical heterodyne technique [2] using high-speed photodiodes (PDs) [3, 4] for THz signal generation, the flexibility and usability of THz photonic components [5] in spectroscopy applications will also be increased. Operating at 1.55 μm wavelength will allow the usage of the available and low-cost telecom optical components in the spectroscopy systems. In addition, using a Schottky barrier diode (SBD) for detecting the transmitted THz signal [6] will allow the development of more flexible and simplified spectroscopy setups. Here, the optoelectronic components and optical devices employed in wireless systems [7] can be also utilized in spectroscopic applications.